This invention relates to a synchronous demodulator for the demodulation of ultrasound burst waves reflected by a moving object, such as, for example, in a cardiotocograph to determine the fetal heart rate.
Fetal monitoring, i.e. monitoring of the fetal condition during gestation and at birth, usually comprises monitoring of the uterus activity (toco) and of the fetal beat-to-beat heart rate (FHR). Among these, the fetal heart rate is the more important parameter as it gives an indication whether the fetus is sufficiently supplied with oxygen. Of course, both parameters may also be used for further diagnostic statements. In particular, the relation between fetal heart rate and labor allows one to evaluate the fetal condition.
To obtain a signal indicative of the fetal heart rate, a so-called fetal scalp electrode ma be applied to the fetal skin. These electrodes are usually spiral electrodes which are screwed the fetal epidermis (see for example U.S. Pat. No. 3,827,428). The electrodes allow very accurate measurements due to the excellent signal quality. Unfortunately, this so-called internal or direct measurement can only be used after rupture of the membranes. Prior to that point in time (in particular, during gestation), indirect methods must be used. These indirect measurements are performed abdominally, e.g., by listening to the fetal heart sound or by measuring the Doppler shift of an ultrasound wave reflected by the moving fetal heart.
The ultrasound technique is the most common one. According to this technique, an ultrasound transducer is placed externally on the pregnant woman's abdomen. The ultrasound signal is received by piezoelectric crystals. The Doppler shift of the reflected ultrasound wave is directly related to the speed of the moving parts (in particular, the walls) of the fetal heart.
Advanced technologies do not use continuous ultrasound waves for the described purpose, but bursts of ultrasound waves instead. The major advantage of the burst technique is, due to the fact that the reflected burst is delayed with respect to the transmitted burst, that a single piezoelectric crystal may be used both as a transmitter and as a receiver. Of course, it is also possible to use a multiplicity of crystals, each of these crystals acting as a transmitter as well as a receiver. A further advantage of the burst technique is that signals reflected in a certain depth of the body may be selected by adjusting a reception period or interval, i.e., by a "time window". By means of this "depth selection", the monitor is able to distinguish between signals resulting from the movements of the fetal heart and other signals, see for example EP-A-204 192.
The received ultrasound wave has to be demodulated in order to obtain the Doppler frequency not as a frequency shift of the high-frequency signal, but rather as a low frequency signal. It is already known to obtain this demodulation by mixing the received ultrasound signal with a square wave high-frequency (which corresponds to the transmitter frequency) signal. According to this method, the received ultrasound wave is switched on and off with the rate of this square-wave signal By this operation, a Doppler signal in the low-frequency range is generated. Still the generated signal contains a lot of other frequency components. Therefore, the signal obtained by mixing the received ultrasound wave with that square-wave signal has to pass a highly selective filter. A major disadvantage of this technique is that the received signal is extremely attenuated thus decreasing the precision of FHR determination. For example, the attenuation caused by the fact that the received signal is switched off for 50% of the available time results in an attenuation of -6 dB. A further, considerably larger attenuation is effected by the interval between two bursts. For example, if we assume a burst length of 5 .mu.s and a repetition rate of the bursts of 3.2 kHz (which corresponds to a repetition interval of 312.5 .mu.s), this results in an attenuation of -36 dB. The overall attenuation is therefore -42 dB. This attenuation is considerable and impairs the quality of the Doppler signal and, therefore, the detected fetal heart rate. This considerable attenuation cannot simply be compensated by an amplifier as such an amplifier generates additional noise.
In advanced technologies, the duration of the burst is adjustable, whereas the burst repetition rate remains constant. In such an application, the known demodulator circuit has the additional disadvantage that the attenuation is not constant when the burst length is varied. This implies additional inaccuracies of the obtained result.